It has been known that vibration, wind pressure, temperature, humidity, static electricity, atmospheric pressure, magnetism, gravitational acceleration, and the like whose effects are mostly ignored in the case where relatively large mass is measured, have an effect on a measurement value as error factors (disturbances) causing measurement errors at the time of measurement of mass, particularly minute mass measurement. In particular, in a device which is capable of minute amount measurements, such as an electromagnetic equilibrium system weighing device (commonly called “analytical balance”) which is capable of weighing 0.1 mg (a weight of approximately 1/10,000 of a weight of a 1-yen coin) or less as a minimum indication, the above-described disturbances are unignorable gross error factors in measurement, and it is necessary to exclude the error factors in calculation of a measurement value by some means.
Here, with respect to the atmospheric pressure, temperature, humidity, and gravitational acceleration among the above-described error factors, changes therein are generated in accordance with motion of heavenly bodies and weather changes, and those changes are relatively gradual, and effects thereof on volume balance, weight balance, and the like are dominant. Therefore, the effects on the performance of the weighing device due to these changes appear as zero-point drifting over a long time. Accordingly, this problem can be solved by a zero-point operation before weighing except for cases of continuous measurement of a same sample for a long time.
On the other hand, for the error factors of vibration, static electricity, and magnetism, methods for actively eliminating or removing the factors have been established, such as providing a vibration removal, vibration isolation, or neutralization mechanism, distancing the source of magnetism, and carrying out magnetic shielding. In addition to eliminating the factors by physical means as such, a program for correcting measurements in response to disturbance factors has also been established.
In contrast to the foregoing error factors, wind pressure and air flow are sudden and often have an effect as an abrupt change in a weighing device capable of minute mass measurement, and thus a correction processing by a program for elimination of measurement errors is virtually impossible. Therefore, there have been proposed various windproof mechanisms for minimizing the effect of wind pressure and air flow on a weighing mechanism section for measuring the mass.
Air fluctuation as error factors in mass measurement, it ranges from a level at which it is possible for a person to sense the air as wind, up to air flow at a slight fluctuation level at which it is practically impossible to sense. Among those, in particular, air flow is unstable, and sustained for a long time in many cases as compared with wind pressure. However, it is possible to cope with air flow outside of a weighing device, for example, flow of heating and cooling air by an air conditioner, flow of discharge air from an air purification system, and the like by means of stopping these devices or distancing the weighing device from these devices, and the like to a certain extent.
On the other hand, in the weighing device itself as shown in FIG. 3A, air (hereinafter called “warm air”) A1 warmed by this electronic substrate and the like rises up to the upper part of the weighing chamber in a weighing chamber 50 by heat generated from an internal electronic substrate and the like, and a stagnant layer A1′ of the warm air A1 is formed sequentially from the upper part toward the lower part of the weighing chamber 50. Further, in this transient state, the air is cooled down by relatively low-temperature air around the weighing chamber, to generate downdraft. The temperature distribution of the air in the weighing chamber 50 is gradually uniformed due to an increase in capacity of the stagnant layer A1′, and therefore, according to this, convection inside the weighing chamber is reduced.
In this way, even in a state in which air flow inside the weighing chamber is stable, which does not have an effect on a measurement value of a sample, relatively intense convection may be generated inside the weighing chamber as described below when the door of the weighing chamber is opened.
When a temperature of the weighing chamber 50 is raised compared to the outside of the device, even if its temperature difference is extremely slight, relatively large convection is generated when the weighing chamber is opened and closed. That is, in the case where a door 50A of the weighing chamber 50 of FIG. 3A is opened to place a sample on a weighing dish 51, even in a case where a temperature inside the weighing chamber 50 is slightly higher than the outside of the weighing chamber 50, as shown in FIG. 3B, the air in the stagnant layer A1′ which has been stagnant in the upper part of the weighing chamber 50 flows out as air A2 along the outerwall of the weighing chamber 50, and air A3 flows into the weighing chamber 50 from the outside of the weighing chamber so as to correspond to the outflow of the air A2. As a result of this, imperceptible convection different from that when the weighing chamber 50 is hermetically closed is generated in the weighing chamber 50, which may lower the reliability of a weighed value of a sample, or may make measurement itself impossible.
Incidentally, it has been confirmed that a weighing error by this air flow (convection) reaches several tens of milligrams at a maximum. This value becomes several tens of dig in a weighing device of 0.1 mg as a minimum indication, which corresponds to 1,000 dig in a weighing device of 1 μg, that is a major cause of measurement error.
In addition, the illustrated case shows a state in the case where the door 50A on the right side toward the front of the weighing chamber 50 is opened. Meanwhile, another door 50B as well is generally formed at a position facing the door 50A in the weighing chamber 50, and in this case, the above-described situation is caused when the door 50B is opened, as a matter of course.
As described above, because it is extremely difficult to process by a program the effects of weighing errors caused in a short time by air fluctuation, in particular, air flow (convection), a variety of windproof mechanisms which physically reduce the effect of air flow on the weighing mechanisms are proposed centering on persons who provide devices for minute amount measurements such as an electronic balance. However, all mechanisms have both merits and demerits.
Patent documents in which windproof structures are proposed are shown below.    Patent Document 1: Japanese Published Unexamined Utility Model Application No. S62-184436    Patent Document 2: Japanese Patent No. 2822671